How to use Photoshop as a neutral density filter
A gradual neutral density filter gives you a gradual transition between light and dark thus giving you the possibility to expose a photo with very light and dark areas correctly. Think of a photo with a white beach and a dark blue sky and you think of a neutral density filter.
Neutral density filters are great, but suppose you forgot yours? Do not worry, within your digital darkroom it is not too difficult to do. I don't have a photo with a white beach and a dark blue sky, and this forum is about SANParks anyway so I am going to use a different photo. Taken in the Sabi Sands, but who is going to cry wolf?
Here is the original scan, I have not done anything yet.
As you can see the weather was lousy and gray, so I now have a underexposed gray photo, not a very exiting sky, and very dark greens. People with extremely good eyes may make out the spoor of an elephant in the riverbedding.
As you can also see the scanner has given me two extra's, a thin scratch on the left, and a blueish belt on the right. Lets get rid of those. (An inbetween tutorial.)
Press CTRL-0 (zero) to fit the photo to your screen. Choose the Healing Brush Tool from your toolbar (The key for it is J). In the part of the toolbar below it is the left on on the top. Set the brushsize to something like 80. Press Alt, the cursor changes to a crosshair, and click on a place with around the same color and texture as the patch you want to "heal". Leftmousebutton depressed wipe over the area to be healed and release the mousebutton. Depending on your systems processing power you see where you wiped as a darker stripe for a moment, don't worry, that will go away in a sec. Touch up as needed, but change the place from where you pick the color when you get to lighter/darker area's. Do difficult areas with a smaller brush and single clicks, not wipes. Made a mistake? CTRL-ALT-Z is your friend.
The photo looks like this now (I left some bits of the stripes on purpose, so you can see the difference.)
Now for the real work, let us make a mask, telling Photoshop what area's may be edited, and which not. The nice thing is that a mask is not a hard on/off situation, but has a gradual transition between on and off. So switch to the Quickmask Mode by hitting Q, or by clicking the marked button in the toolbar.
OK, it is on, now to reset the tool to the default values of black & white, just to make sure they are set. Press D on your keyboard. Done. (Easy what?)
Now pick the Gradient Tool by pressing G or, if the bucket is visible select it from the toolbar by clicking and holding the little triangle to the right of the bucket and then selecting Gradient. Make sure the gradient is set to linear.
To put the mask in place put your mouse (a little cross at the moment) at the top of the photo and move straight down to the horizon. The air will become red, and is protected against all edits.
Switch back to normal editing mode (Q) and you will see the selection. Note that the selection does not stop at the horizon.
Now for the real work, adjust the Levels (CTRL-L or Menubar, Image, Adjustments, Levels), as needed. (This works the same way as the Levels I have written about earlier, see here
When you are happy with the colors it is time to do a little USM (Unsharp Mask). We do not want to sharpen the sky, but the sky is protected now so we can safely do that now. Go to Menubar, Filter, Sharpen, Unsharp Mask, and do a bit of sharpening. (Like 100%, Radius 0,5 and Threshold 0, depending on your photo.)
Bottom of the photo done, now invert the selection (CTRL-SHIFT-I) and Level the sky. (CTRL-L again.) If you were to peek at the Quickmask Mode you would see this:
For the sake of the tutorial I have overdone the darkening of the sky.
Next was the branch in the top of the photo, which was way to dark after the darkening of the sky.
That was a simple matter of selecting the leaves using the magic wand and doing Levels on them. (The branch needs multiple selections, no problem, keeping SHIFT depressed you add to the selection.)
When you have the maskingprocess firmly in your head, you will probably be substracting the branch from the mask. That way you are really changing air and trees/grass seperately, and the branch will stay a part of the photo.
This came out, still not a good photo, but better than it was I think.
To compare between original and filtered one here is the original with a part of the finished photo in between: